The acid-stable protease inhibitor of human bronchial mucus will be isolated and characterized and its relationship to the inter-alpha-trypsin inhibitor and other inhibitors of serum will be determined. The site of synthesis and interaction with proteinases will be studied. The trypsin inhibiting activity, the concentration of the acid-stable inhibitor and that of alpha 1-antitrypsin in sputum and bronchial mucus will be determined in patients with chronic obstructive pulmonary disease and controls. Proteinase-inhibitor complexes will be analyzed as to their composition and remaining enzyme activity. These experiments are done in an attempt to elucidate the role of protease inhibitors in the bronchial secretions and to reveal possible factors predisposing to obstructive lung disease.